ICs (integrated circuits) are generally made on a thin and circular silicon wafer. The transistors and wiring of an IC are conventionally made from many mask layers built on top of one another. Each successive mask layer may have a pattern that is defined using a mask. A mask is the shape used for processing features in a particular process step of integrated circuit production. When a mask layer is defined using a mask chosen or provided by a customer, the mask layer is programmed or customized.
An ASIC (application specific integrated circuit) is an IC designed specifically for a particular application or use. There are three categories of ASICs in terms of programmability of mask layers: fully programmable ASICs, non-programmable ASICs, and semi-programmable ASICs. In a fully programmable ASIC, all mask layers are programmed or customized. A typical example of a fully programmable ASIC is a cell-based ASIC (CBIC). In a non-programmable ASIC, none of the mask layers are programmed or customized, and all mask layers are prefabricated by an ASIC vendor. A typical example of a non-programmable ASIC is a field-programmable gate array (FPGA). In a semi-programmable ASIC, some, but not all, mask layers are programmed. A typical example of a semi-programmable ASIC is a gate-array-based ASIC. Semi-programmable ASICs blending the performance found in the CBIC with the time-to-market provided by the FPGA are vigorously pursued by the ASIC vendor because such ASICs may reach out to a growing class of customers that are unwilling to pay for the CBIC but find the FPGA too constraining.
Conventionally, a metal layer of ASICs has been formed by blanket depositing a layer of metal on a substantially planar insulating surface. Portions of the metal layer are subsequently removed in an etching step to form the resulting metal conductors. Copper is preferred due to lower resistivity and better electromigration resistance. Unfortunately, copper may be difficult to etch in such a process. Traditional ASICs have therefore employed a more resistive metal, aluminum (or aluminum alloy), for the metal layers. For the metal inside contact holes and vias formed within the insulation layers, tungsten has been conventionally used.
Recently, a “damascene” process has been developed whereby copper may be used for the metal layers. Rather than blanket depositing metal on a substantially planar insulating substrate and then etching away parts of the metal layer to leave the conductors, trenches may be formed in an insulating material. A layer of metal may be blanket deposited over the entire surface of the insulating substrate such that the trenches are filled. Next, chemical or mechanical polishing may be used to planarize the integrated circuit surface and thereby polish away all the metal that is not in the trenches. The result may be metal conductors disposed in trenches. However, the copper process may cost significantly more than the traditional aluminum process.
Thus, it would be desirable to provide a semi-programmable ASIC using both copper (or copper alloy) and aluminum (or aluminum alloy) for the metal layers.